Allow n_added to accept vectors

[njtierney]

So n_added = c(20, 40, 60)

Would fit the optimisation model 3 times, with different n_added.

It would also store the results in a list

[njtierney]

Just to be clear, the input would be this

max_coverage(existing_facility = wifi,
             proposed_facility = property,
             user = stops,
             distance_cutoff = 92,
             n_added = c(100, 200, 300))